Here is an informative piece on injection moulding of rubber products. This article is sent to me by Dr.S.N.Chakravarty, President – Elastomer Technology Development Society and Ex-Chairman, Indian Rubber Institute (IRI).

Moulded rubber products, including rubber to metal bonded items, are manufactured by following process world-wide. Choice of process depends on different factors like Product Geometry, Type of Polymer, Volume, Cost Vs Market Price realization, Manpower & Space etc.

1. Compression Moulding
2. Transfer Moulding
3. Injection Moulding

All the process are well established and widely used. Each process has certain advantages & disadvantages. To get optimum result, you all need suitably developed compound & manpower besides right machine & process.

Compound is developed to meet requirement of laid down specification including special requirement (e.g. dynamic fatigue, heat development, tear resistance, load deflection, etc.) and retention of properties on ageing for specific period at elevated temperature. The later indicates approximate idea for product performance & life. All these obviously have bearing on cost factor.

For different moulding process three major properties or compound behavior is to be considered – Compound Flow Characteristics, Mooney Viscosity and Scorch Characteristics. Each process calls for adjustment of these parameters for suitable working.

Moulding:

Primary purpose of moulding is to provide shape & size of the product. Porosity & Knit in the product are to be avoided. Porosity is caused by moisture in the compound & moulding pressure and Knit by improper compound flow.

In case of Compression moulding, preparation of Blank (shape, size, weight) plays important role to control mould flow / avoid air entrapment and excess flash.

This point is not necessary for Transfer or Injection moulding where feeding system & wastage of semi-scorched compound are more important factor.

Basics of Injection Moulding

Injection Moulding is an important processing technique for converting elastomeric materials into final products. The injection moulding machine automatically performs: raw material feeding, heating, polymer mix plasticization, and mould injection; all operations are performed under controllable conditions of temperature, time, speed, and pressure.

Some peculiar problems arising during injection moulding are associated with the drastic change in rubber compound rheological properties which occur upon vulcanization. In particular, during the filling stage, the rubber compound in the mould-filling process. This is a phenomenon that may lead to the loss of processability of the rubber compound and incomplete mould filling.

Rubber compound or thermosets go through an irreversible chemical change during the forming process. Thus, the critical issue in modeling the injection moulding process for such polymeric materials, is developing a viscosity model that can accurately describe the reheological and chemo-rheological behavior.

As Injection Moulding of Rubber compounds becomes an increasingly important moulding process, an understanding of the nature of coupling the rheological behavior with cure kinetics of rubber compounds and the process itself becomes paramount. Effective control of product quality, as well as the ability to optimize moulding conditions, will depend on a further development of such an understanding.

During Injection Moulding, the Rubber compounds are subjected to high shear rates and as a result experience a thermal history leading to chemical reaction (cross linking). It is recognized that the processibility of a rubber compound is predominantly determined by its viscosity which is balanced by tow effects: the drastic decrease in viscosity with increasing shear rate: and a significant increase in viscosity following the onset of vulcanization. Thus, different rubber compounds not only display different rheological properties, by also possess different chemo-rheological behavior.

It is seen that an increase in the inlet temperature does not significantly reduce the cycle time needed to reach a specified cure level. One cannot further raise the inlet temperature to achieve a shorter cycle time due to the possibility of curing in the barrel. Therefore, the distribution of state-of cure in the moulded rubber articles is strongly dependent upon the mould temperature and less dependent upon the inlet temperature.

It is interesting to know how the injection speeds affects the state-of-cure achieved during the same mould cycle duration. For this purpose experiments were conducted in which the injection speed was set  at a definite speed. At this injection speed the calculation shows that there is no cure during the filling stage.

A comparison is made between the predicted and measured gap-wise distribution of state-of-cure in mouldings obtained where cure occurs during filling by  lower injection speed, and where it does not occur during filling at higher injection speed. In the later case, the ultimate state-of-cure achieved in the cure stage is the overall state-of-cure achieved during entire moulding cycle.

In the case when cure occurs during filling, the overall state –of-cure for the entire moulding cycle is the state-of-cure at the end of filling plus the state-of-cure further developed during the cure. Surprisingly, both experimental and predicted gap-wise results show that a higher stat-of-cure is obtained at higher injection speed for the same moulding cycle.

This can be explained by the fact that the rubber material gets into the cavity much faster and therefore has a much longer time to cure in the cavity.

The development of the state-of-cure in the post-filling stage indicated that the mould temperature and injection speed strongly affected the evaluation of state-of-cure within the moulded parts. The effects of the inlet temperature on the state-of-cure was found to be insignificant.

With the increasing demands for high quality and productivity in injection moulding, it has become more important to keep consistently constant dimensional precision. Various properties of polymer such as melt fluidity and density are dependent highly upon temperature. Therefore the consistency in quality is primarily dominated by the temperature variations of mould wall and / or polymer in the barrel.

Fluctuations of these temperatures, however, are indispensable to some extent, due to the heat exchange among polymer, mould, machine and environment, and transient temperature change at the start-up, etc. An adaptive holding pressure control system was developed to get constants product weight by changing holding pressure adapted to the temperature variations.

In Part 2, you will look more deeply into Injection moulding process.

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